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Abstract:

At least one embodiment of the invention relates to a protective circuit
for protection of an appliance, in particular of an electric motor,
against thermal overloading. In at least one embodiment, the protective
circuit includes a first and a second terminal for connecting a
temperature detection element; a detection unit for detecting whether a
resistance value of the temperature detector element which is connected
between the first and the second terminals, is in a first or a second
resistance range; a short-circuit detector for identifying whether there
is a short circuit between the first and the second terminals; signaling
outputs for emitting signal messages to an evaluation unit when the
detected resistance value is in the second resistance range and/or when a
short circuit has been identified by the short-circuit detector; and a
third terminal, which is connected to the second terminal via a
predetermined resistance.

Claims:

1. A protective circuit for protection of an appliance against thermal
overloading, comprising:a first and a second connection to connect a
temperature detector element;a detection unit to detect whether a
resistance value of the temperature detector element, connected between
the first and the second connection, is in a first or a second resistance
range;a short-circuit detector to detect whether a short circuit is
present between the first and the second connection;signaling outputs to
emit message signals to an evaluation unit at least one of when the
detected resistance value is in the second resistance range and when a
short circuit is detected by the short-circuit detector; anda third
connection, connected to the second connection via a resistance.

2. The protective circuit as claimed in claim 1, wherein the resistance is
chosen such that the short-circuit detector does not identify a short
circuit even when there is a short circuit between the first and the
third connection.

3. The protective circuit as claimed in claim 1, wherein the resistance is
in the first resistance range.

4. The protective circuit as claimed in claim 1, wherein resistance values
in the first resistance range are relatively lower than resistance values
in the second resistance range.

5. The protective circuit as claimed in claim 1, wherein the protective
circuit is for protection of an electric motor against thermal
overloading.

6. The protective circuit as claimed in claim 2, wherein resistance values
in the first resistance range are relatively lower than resistance values
in the second resistance range.

7. A protective circuit for protection of an appliance against thermal
overloading, comprising:first and second means for connection of a
temperature detector element;detection means for detection of whether a
resistance value of the temperature detector element, connected between
the first and the second means, is in a first or a second resistance
range;short-circuit detector means for detecting whether a short circuit
is present between the first and the second means;means for emitting
message signals to an evaluation unit at least one of when the detected
resistance value is in the second resistance range and when a short
circuit is detected by the short-circuit detector means; andthird means
for connection, connected to the second means via a resistance.

8. The protective circuit as claimed in claim 7, wherein the resistance is
chosen such that the short-circuit detector means does not identify a
short circuit even when there is a short circuit between the first and
the third means.

9. The protective circuit as claimed in claim 7, wherein the resistance is
in the first resistance range.

10. The protective circuit as claimed in claim 7, wherein resistance
values in the first resistance range are relatively lower than resistance
values in the second resistance range.

11. The protective circuit as claimed in claim 7, wherein the protective
circuit is for protection of an electric motor against thermal
overloading.

12. The protective circuit as claimed in claim 8, wherein resistance
values in the first resistance range are relatively lower than resistance
values in the second resistance range.

Description:

PRIORITY STATEMENT

[0001]This application is the national phase under 35 U.S.C.§371 of
PCT International Application No. PCT/DE2006/001558 which has an
International filing date of Sep. 6, 2006, which designated the United
States of America, the entire contents of each of which are hereby
incorporated herein by reference.

FIELD OF THE INVENTION

[0002]At least one embodiment of the invention generally relates to a
protective circuit for protection of an appliance, in particular an
electric motor, against thermal overloading.

BACKGROUND

[0003]In addition to an overload relay, which identifies overloading of a
motor on the basis of the current drawn by the motor, thermistor
protective circuits are also used for protection against thermal motor
overloading, and these circuits detect and evaluate the temperature of a
motor winding. Various temperature detector elements are used for this
purpose, for example temperature sensors, in particular PTC thermistors
(PTC: positive temperature coefficient), PT100 sensors and KTY sensors,
whose electrical resistance varies as a function of the detected
temperature. In particular, the resistance of a temperature detector
element such as this is in a first range for as long as the motor winding
is in a temperature range in which there are no problems. The second
resistance range, whose resistance values are greater, for example, than
those of the first resistance range, then corresponds to a temperature
above a temperature threshold value.

[0004]Furthermore, so-called thermo-click elements, which are also
referred to as temperature monitors, can also be used as temperature
detector elements. These are generally represented by simple switches
which are normally designed using a bimetallic strip, which is closed in
the first temperature range and is opened in a second temperature range,
that is to say above the temperature threshold value. This means that the
resistance changes from virtually 0Ω to an infinite resistance.

[0005]In the case of the first-mentioned temperature detector elements,
the temperature sensors, overloading of the motor is evaluated by way of
the resistance change of the temperature detector elements within
predefined value ranges. In contrast, in its normal field of application,
the thermo-click element opens the circuit through a motor winding on
reaching the temperature threshold value. In contrast, temperature
sensors are evaluated by means of a protective circuit which essentially
determines the electrical resistance of the relevant temperature detector
elements and emits a message signal to a switch, to an evaluation circuit
or the like, thus, for example, initiating a shutdown of the appliance or
starting some other predetermined function when a thermal overload is
identified as a function of the measured temperature.

[0006]Protective circuits such as these are frequently likewise provided
with short-circuit detectors, which likewise result in the corresponding
message signal being emitted and thus, for example, initiate a shutdown
of the appliance to be protected when a short circuit occurs in the
sensor circuit. Without this short-circuit identification, it is no
longer possible to identify a thermal overload when a conductor
short-circuit occurs, since it is no longer possible to identify an
increase in the resistance of the temperature sensor. Since short-circuit
identification is necessary in order to obtain a specific license, for
example ATEX (that is to say the appliance can be used for protection of
motors in explosion-hazard areas), it is desirable to use a protective
circuit such as this with short-circuit identification.

[0007]If one wishes to use a protective circuit such as this for
temperature sensors of the abovementioned type, for example PTC
thermistors, PT100 sensors and KTY sensors, as well as for thermo-click
elements, the closed state of the thermo-click element is identified in
the normal state, that is to say at temperatures in the first temperature
range, as a short circuit through the protective circuit, which then
initiates a shutdown of the appliance.

[0008]In consequence, until now, it has not been possible to use a
protective circuit which is provided for temperature sensors in
conjunction with a thermo-click element at the same time.

[0009]Until now, only protective circuits which evaluate temperature
sensors without short-circuit identification have been known, which are
therefore also suitable for evaluation of thermo-click elements, or
circuits for thermistors which have short-circuit identification and are
therefore not suitable for evaluation of thermo-click elements. In
addition, evaluation circuits are already known in which it is possible
to use adjustment parameters or configuration to choose between operation
with short-circuit identification and operation without short-circuit
identification.

SUMMARY

[0010]At least one embodiment of the present invention provides a
protective circuit for protection of an appliance against thermal
overloading, in which thermal overloading can be detected not only by
temperature sensors but also alternatively by thermo-click elements.

[0011]At least one embodiment of the invention provides an evaluation
circuit which makes it possible to choose between operation with
short-circuit identification and operation without short-circuit
identification, without any additional adjustment elements or
configuration being required for this purpose.

[0012]According to at least one embodiment of the invention, a protective
circuit is provided for protection of an appliance, in particular an
electric motor, against thermal overloading. The protective circuit
comprises a first and a second connection for connection of a temperature
detector element, a detection unit for detection of whether a resistance
value of a temperature detector element which is connected between the
first and the second connection is in a first or in a second resistance
range, and a short-circuit detector for identification of whether there
is a short circuit between the first and the second connection.
Furthermore, signaling outputs are provided for emission of message
signals to an evaluation circuit when the detected resistance value is in
the resistance range or when a short circuit is identified by the
shirt-circuit detector, or both have occurred. A third connection is also
provided, and is connected to the second connection via a predetermined
resistance.

[0013]The protective circuit according to at least one embodiment of the
invention now makes it possible to connect both temperature sensors and
thermo-click elements in such a way that thermal overloading can be
detected both by temperature sensors and by a thermo-click element, and
it is nevertheless possible to identify a short circuit between the first
and the second connection, even when a thermo-click element is connected
to the protective circuit for temperature detection. It is thus possible
to operate the protective circuit either with a temperature sensor whose
electrical resistance depends on the detected temperature, or with a
thermo-click element. Furthermore, passive switching of the protective
circuit can be achieved, without having to implement other measures in
the overall system, by means of an externally applied short circuit
between the first and the third connection.

[0014]Furthermore, the predetermined resistance can be chosen such that no
short circuit is identified by the short-circuit detector even when there
is a short circuit between the first and the third connection. This
prevents the thermo-click element initiating the short-circuit
identification and emitting the message signal, and thus for example
shutting down the appliance, in the normal state, that is to say when no
overload is detected, when this thermo-click element is connected to the
protective circuit.

[0015]The predetermined resistance is preferably in the first resistance
range. This first resistance range is defined by the detection unit as
the resistance range which corresponds to a temperature range in which no
overload has occurred.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]One example embodiment of the present invention will be explained in
more detail in the following text with reference to the attached
drawings.

[0017]FIG. 1 shows a block diagram of a protective circuit 1 according to
an embodiment of the invention.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

[0018]FIG. 1 shows a block diagram of a protective circuit 1 according to
an embodiment of the invention having a first connection 2 and a second
connection 3, in order to connect a temperature sensor, such as a PTC
thermistor, a PT100 sensor, a KTY sensor or some other temperature sensor
as a temperature detector element whose electrical resistance is a
function of the detected temperature and in particular has a positive
temperature gradient, that is to say the electrical resistance increases
as the temperature rises.

[0019]The temperature sensor 4 is connected via connecting lines 5 to the
first and the second connection 2, 3 of the protective circuit 1. The
protective circuit 1 has a detection unit 6 which detects whether a
resistance value of the temperature sensor 4 which is connected between
the first and the second connection 2, 3 is in a first or in a second
resistance range. The first resistance range is preferably a resistance
range with low resistance values, and the second resistance range is
preferably a range with higher resistance values, such that the detection
unit identifies whether the temperature at the temperature sensor 4 is in
the first or in the second temperature range.

[0020]If it is detected that the resistance of the temperature sensor 4 is
in the first resistance range, then no thermal overload has occurred,
because the temperature is within the first temperature range, in which
there are no problems. If the resistance value of the temperature sensor
4 is in the second resistance range, thermal overloading is detected and
a first message signal AS1 is emitted via a first signaling output 8.

[0021]Furthermore, a short-circuit detector 10 is provided which is
likewise connected to the first and the second connection of the
protective circuit 1 and which detects whether a short circuit has
occurred between the first and the second connection, that is to say
whether the electrical resistance between the first and the second
connection 2, 3 is 0 ohms or virtually 0 ohms. It is therefore possible
to identify a short circuit between the connecting lines 5 and the
temperature sensor 4, and a second message signal AS2 is generated as a
function of this, and is emitted to the second signaling output 9. Both
signaling outputs 8, 9 are connected to an evaluation unit 14 which, for
example, causes a motor to be shut down as a function of a fixed or
configurable function, via its output 15. In one simple embodiment, for
example, the evaluation unit 14 comprises an OR gate for logically
linking the two message signals.

[0022]If one now wishes to use a thermo-click element 11 instead of the
temperature sensor 4 and this were to be connected to the first and the
second connection 2, 3 of the protective circuit 1, then this would lead
to the short-circuit detector 10 identifying a short circuit, since the
thermo-click element 11 essentially represents a switch which is closed
below a temperature threshold value. This means that, if the temperature
which is detected by the thermo-click element 11 is below a predetermined
temperature threshold value, then the thermo-click element 11 has
virtually no electrical resistance, which would incorrectly be detected
as a short circuit when connected to the first and the second connection
2, 3 of the protective circuit 1.

[0023]In order to make it possible for the protective circuit 1 to be
operated with thermo-click elements as well, a third connection 12 is
provided and is connected to the second connection via a resistance
element 13 which has a predetermined resistance value. An embodiment of
the invention now provides that, when a thermo-click element 11 is being
used as a temperature detector element, this is connected between the
first and the third connection 2, 12 in the protective circuit 11. In
this case, the thermo-click element 11 is connected in series with the
resistance element 13 in such a way that, even when the thermo-click
element 11 is in the closed state, the short-circuit detector 10 cannot
detect a short circuit.

[0024]If the temperature threshold value is exceeded, then the
thermo-click element 11 opens, and the connection between the first
connection and the third connection, that is to say between the first and
the second connection, is broken. A very high resistance is detected
which, as described above, is in the second resistance range, as a result
of which the detection unit 6 identifies thermal overloading and emits a
corresponding message signal AS1 at the signal output 8.

[0025]It is also possible to additionally completely deactivate the
protective circuit 1 by connecting a wire link between the first
connection and the third connection, which would not be possible in the
case of conventional protective circuits because of the short-circuit
identification, since this would cause the message signal to be emitted
immediately. It is therefore possible to dispense with one additional
switching element within the protective circuit 1, by which the
protective circuit 1 can be deactivated while it is not intended to use a
temperature detector element for detection of thermal overloading.

[0026]The resistance value of the resistance element 13 is preferably
chosen such that the resistance value is within the first resistance
range, such that, when the thermo-click element 11 is closed, the
protective circuit 1 detects a resistance value which indicates that no
thermal overloading has occurred. For example, the predetermined
resistance value of the resistance element 13 may correspond to the
resistance value of a temperature sensor 4 which can be used at room
temperature (provided that a thermal overloading situation does not occur
at room temperature).

[0027]This makes it possible to produce a protective circuit 1 which is
suitable for operation either with a temperature sensor whose resistance
value depends on the temperature or with a thermo-click element, that is
to say a temperature switch and in which, furthermore, it is also
possible to identify a short circuit when using a temperature sensor, in
order to comply with the specified requirements, for example those for
ATEX licensing.

[0028]Example embodiments being thus described, it will be obvious that
the same may be varied in many ways. Such variations are not to be
regarded as a departure from the spirit and scope of the present
invention, and all such modifications as would be obvious to one skilled
in the art are intended to be included within the scope of the following
claims.